<html><body><div style="color:#000; background-color:#fff; font-family:HelveticaNeue, Helvetica Neue, Helvetica, Arial, Lucida Grande, sans-serif;font-size:12px"><div id="yui_3_16_0_1_1443723276430_4901">Hi Bo,</div><div id="yui_3_16_0_1_1443723276430_4902"><br></div><div id="yui_3_16_0_1_1443723276430_5005" dir="ltr">in addition to what Jack said, the following paper may be of help:</div><div id="yui_3_16_0_1_1443723276430_5007" dir="ltr"><br></div><div id="yui_3_16_0_1_1443723276430_4957" class="">
Peter Lynch, 1989: Partitioning the Wind in a Limited Domain. <i class="" id="yui_3_16_0_1_1443723276430_4959">Mon. Wea. Rev.</i>, <b class="" id="yui_3_16_0_1_1443723276430_4961">117</b>, 1492–1500.
</div>
<div id="yui_3_16_0_1_1443723276430_4963" class=""><div id="yui_3_16_0_1_1443723276430_5009" dir="ltr">
doi: <a class="" id="yui_3_16_0_1_1443723276430_4965" href="http://dx.doi.org/10.1175/1520-0493%281989%29117%3C1492:PTWIAL%3E2.0.CO;2">http://dx.doi.org/10.1175/1520-0493(1989)117<1492:PTWIAL>2.0.CO;2</a>
</div></div><div id="yui_3_16_0_1_1443723276430_5011"><br></div><div id="yui_3_16_0_1_1443723276430_5013">Good luck!</div><div id="yui_3_16_0_1_1443723276430_5015">Dave.<br></div><div id="yui_3_16_0_1_1443723276430_5017"><br></div><div id="yui_3_16_0_1_1443723276430_5019">------------------------------</div><div class="qtdSeparateBR"><br><br></div><div style="display: block;" id="yui_3_16_0_1_1443723276430_4544" class="yahoo_quoted"><div id="yui_3_16_0_1_1443723276430_4543" style="font-family: HelveticaNeue, Helvetica Neue, Helvetica, Arial, Lucida Grande, sans-serif; font-size: 12px;"><div id="yui_3_16_0_1_1443723276430_4542" style="font-family: HelveticaNeue, Helvetica Neue, Helvetica, Arial, Lucida Grande, sans-serif; font-size: 16px;"><div id="yui_3_16_0_1_1443723276430_4541" class="y_msg_container"><br>Message: 4<br>Date: Thu, 1 Oct 2015 14:50:16 +0200<br>From: Jack Ogaja <<a ymailto="mailto:jack.ogaja@gmail.com" href="mailto:jack.ogaja@gmail.com">jack.ogaja@gmail.com</a>><br>Subject: Re: [ncl-talk] Calculate divergent wind component on a<br> non-global grid<br>To: "Dong, Bo" <<a ymailto="mailto:bdong@albany.edu" href="mailto:bdong@albany.edu">bdong@albany.edu</a>><br>Cc: "<a id="yui_3_16_0_1_1443723276430_4860" ymailto="mailto:ncl-talk@ucar.edu" href="mailto:ncl-talk@ucar.edu">ncl-talk@ucar.edu</a>" <<a ymailto="mailto:ncl-talk@ucar.edu" href="mailto:ncl-talk@ucar.edu">ncl-talk@ucar.edu</a>><br>Message-ID: <<a ymailto="mailto:B08A72AF-888A-4562-AF41-026E5694B4A3@icloud.com" href="mailto:B08A72AF-888A-4562-AF41-026E5694B4A3@icloud.com">B08A72AF-888A-4562-AF41-026E5694B4A3@icloud.com</a>><br>Content-Type: text/plain; charset="windows-1252"<br><br>Hi Bo,<br><br>1) Assuming that your limited area grid is a Regular, Fixed or Fixed-Offset(e.g. with rotated coordinates) grid, you can use the function ?uv2dv_cfd()? to calculate the divergence(div). However, you have to be careful with the definitions/positions of the wind components on the grid - whether staggered or non-staggered.<br><br>2) Calculating velocity potential(Chi) and divergent wind component in a limited area domain is however an endeavor.<br> i) First, you cannot define wind field from velocity potential(Chi) alone, you also need the stream-function(Psi). That means you have to calculate the vorticity(vr) as well using the function ?uv2vr_cfd()? assuming conditions in (1)<br> i.e. from Helmholtz theorem, you end up with two elliptic equations<br> Laplace(Psi)= vr (a)<br> Laplace(Chi)= div (b)<br> ii) In a global domain equations (a) and (b) are ?not bound to? any boundary condition. However, in a limited area domain the equations are coupled by the boundary conditions and should be solved simultaneously.<br> i.e. if you consider wind field(V) decomposed into non-divergent and irrotational components as,<br> V=k x grad(Psi)+grad(Chi) (c)<br>then, for Chi and Psi to satisfy equation (c) at the boundaries you need the normal and tangential wind components each of which involves derivatives of both Chi and Psi. <br><br>Equations (a) and (b) are commonly solved using successive over-relaxation(SOR) method, but any elliptic solver can be applied with a properly defined boundary condition.<br><br>I can only suggest that you write your own solver for equations (a) and (b) in c++ or fortran or whatever program language you deem suitable and call it in ncl using WRAPIT script - <a id="yui_3_16_0_1_1443723276430_4861" href="https://www.ncl.ucar.edu/Document/Tools/WRAPIT.shtml." target="_blank">https://www.ncl.ucar.edu/Document/Tools/WRAPIT.shtml. </a>If you don?t want to go all the way to write your own program, there are even efficient direct solver subroutines specifically ?hwscrt()? and ?hwsssp()? from the ?FISHPACK? library - <a href="https://www2.cisl.ucar.edu/legacy/fishpack/documentation," target="_blank">https://www2.cisl.ucar.edu/legacy/fishpack/documentation, </a>which you can ?wrap? in ncl as well.<br><br>Jack. <br><br>> On 01 Oct 2015, at 03:57, Dong, Bo <<a id="yui_3_16_0_1_1443723276430_4890" ymailto="mailto:bdong@albany.edu" href="mailto:bdong@albany.edu">bdong@albany.edu</a>> wrote:<br>> <br>> Hi All,<br>> <br>> As I'm trying to calculate the divergence and divergent wind component, I found a few functions such as dv2uvf, dv2ufg. However, all these functions require that U and V wind on a global grid. I'm wondering that for regional data, how can I compute the divergence, velocity potential and divergent wind component on a non-global grid? Thanks.<br>> <br>> regards,<br>> Bo<br>> _______________________________________________<br>> ncl-talk mailing list<br>> <a ymailto="mailto:ncl-talk@ucar.edu" href="mailto:ncl-talk@ucar.edu">ncl-talk@ucar.edu</a><br>> List instructions, subscriber options, unsubscribe:<br>> <a href="http://mailman.ucar.edu/mailman/listinfo/ncl-talk" target="_blank">http://mailman.ucar.edu/mailman/listinfo/ncl-talk</a><br><br></div> </div> </div> </div></div></body></html>